Drywall air gap question...

Hello! I'm planning on adding 2 layers of 5/8" drywall on the ceiling of my studio construction. I'm wondering if it's better to separate the layers with strips of 1 x 2 to get an air gap between, or just stack the layers together. Seems like the extra little airgap would be good but I'm often surprised by what tests the best. any thoughts? Thanks!

I'm wondering if it's better to separate the layers with strips of 1 x 2 to get an air gap between, or just stack the layers together.

That would actually be a bad idea! As with many things in acoustics, it isn't intuitive at all, at first glance, but the explanation is simple. Any air space trapped between two massive surfaces is a resonant system. The resonance can easily be calculated with a simple equation, and at that specific frequency of resonance, it does not isolate at all. Even worse, it can actually amplify the sound, so that it would be louder on the other side, since the system itself is resonating. This is the principle that MSM walls are based on, except that they are tuned such that the resonant frequency is one octave lower than the lowest frequency you need to isolate, therefore ensuring that the resonance does not happen in the part of the audio spectrum that you are trying to isolate. To "tune" the frequency low enough, you need lots of mass on the leaves, and a very large gap between them, on the order of several inches. If the gap is just 1" or so, then the resonant frequency will be very high up the scale, and well into the normal audio spectrum, so your wall or ceiling would isolate WORSE if you did that, since it would actually transmit that frequency (and the ones around it for about an octave on either side...!) to the other side. Worse still ( :shock: ), assuming that you are building a normal 2-leaf wall or ceiling, adding that extra spacer in between two layers of drywall makes your wall into a three-leaf system, which no longer has just one resonant frequency: it now has two resonant frequencies, and BOTH of them are higher than it would have been if you had just a built a 2-leaf wall with the same materials and same total thickness. In other words: NO! Don't do that. Bad idea. It will have the opposite effect of what you had hoped for. Far better would be to put a layer of Green Glue in between the two layers of drywall. - Stuart -

Thanks. I figured it would be something like that. A contractor friend of mine was suggesting it but the specific science of sound transmission isn't always intuitive. I appreciate your feedback.